One construction of a slant plate type compressor, particularly a wobble plate compressor, with a variable capacity mechanism which is suitable for use in an automotive air conditioner is disclosed in U.S. Pat. No. 3,861,829 issued to Roberts et al. Roberts et al. '829 discloses a wobble plate type compressor which has a cam rotor driving device to drive a plurality of pistons. The slant or incline angle of the slant surface of the wobble plate is varied to change the stroke length of the pistons which changes the displacement of the compressor. Changing the incline angle of the wobble plate is effected by changing the pressure difference between the suction chamber and the crank chamber in which the driving device is located.
In such a prior art compressor, the slant angle of the slant surface is controlled by the pressure in the crank chamber. Typically this control occurs in the following manner. The crank chamber communicates with the suction chamber through an aperture and the opening and closing of the aperture is controlled by a valve mechanism. The valve mechanism generally includes a bellows element and a needle valve, and is located in the suction chamber so that the bellows element operates in accordance with changes in the suction chamber pressure.
In the above compressor, the pressure of the suction chamber is compared with a predetermined value by the valve mechanism. However, when the predetermined value is below a certain critical value, there is a possibility of frost forming on the evaporator in the refrigerant circuit. Thus, the predetermined value is usually set higher than the critical value to prevent frost from forming on the evaporator.
However, since suction pressures above this critical value are higher than the pressure in the suction chamber when the compressor operates at maximum capacity, the cooling characteristics of the compressor are inferior to those of the same compressor without a variable displacement mechanism.
Roberts et al. '829 discloses a capacity adjusting mechanism used in a wobble plate type compressor. As is typical in this type of compressor, the wobble plate is disposed at a slant or incline angle relative to the drive axis, nutates but does not rotate, and drivingly couples the pistons to the drive source. This type of capacity adjusting mechanism, using selective fluid communication between the crank chamber and the suction chamber can be used in any type of compressor which uses a slanted plate or surface in the drive mechanism. For example, U.S. Pat. No. 4,664,604 issued to Terauchi discloses this type of capacity adjusting mechanism in a swash plate type compressor. The swash plate, like the wobble plate, is disposed at a slant angle and drivingly couples the pistons to the drive source. However, while the wobble plate only nutates, the swash plate both nutates and rotates. The term slant plate type compressor will therefore be used to refer to any type of compressor, including wobble and swash plate types, which use a slanted plate or surface in the drive mechanism.
A signal controlled compressor solenoid valve in combination with a pressure actuated bellows valve is disclosed in U.S. patent application Ser. No. 076,282 which corresponds to Japanese Utility Model Application No. 61-111994 to improve cooling characteristics and temperature control in the passenger compartment.
In a starting so-called "cool down" stage of an air conditioning system including such a compressor for initially cooling the passenger compartment, the second valve control device works to connect the crank chamber to the suction chamber due to a heat load on the evaporator of the air conditioning system being exceedingly above a single predetermined value. Once the heat load drops to the same predetermined value, the second valve control device closes the valve and only may reopen the valve if the heat load exceeds that single predetermined value which will normally only occur after the air conditioning system has been turned off and then restarted after a certain time period. Once the second valve control device closes the second valve, the first valve control device solely controls the capacity of the compressor.
The air conditioning system including the above mentioned variable displacement mechanism has no problem in a "cool down" stage when cooling recirculated room air.
However, in a "cool down" stage with fresh air intake, i.e., cooling fresh air which is brought into the room, the above mentioned air conditioning system has certain drawbacks.
Referring to FIG. 9, the cool down characteristic of the prior art air conditioning system in a fresh air intake situation is shown. In FIG. 9, a solid line, a dotted line and a dashed line show pressure of an evaporator outlet portion, pressure of a compressor suction chamber and a room (passenger compartment) temperature, respectively. In the cool down stage, the second valve control device works to connect the crank chamber to the suction chamber causing maximum displacement of the slant plate of a slant plate type compressor, so that the room temperature, the pressure in evaporator outlet portion and the pressure in the suction chamber fall quickly. When the pressure in the evaporator outlet portion falls to the single predetermined value P1 that is the lower most point before frost forms on the evaporator surface, the second valve control device closes the second valve (time t.sub.1 elapsed). After time t.sub.1, the first valve control device solely controls the displacement of the compressor slant plate and maintains the suction chamber pressure slightly above P1. Immediately after time t.sub.1, the heat load is still large so that a large amount of refrigerant gas flows from the evaporator to the suction chamber. As a result, some pressure loss occurs between the evaporator outlet portion and the suction chamber which makes the pressure of the evaporator outlet portion quickly rise. The quick pressure rise in the evaporator outlet portion causes inefficient heat exchange which in turn causes the room temperature to quickly rise.
Furthermore, when the above mentioned air conditioning system incorporates a mechanical thermal expansion valve which maintains super heat values associated with the evaporator outlet portion generally constant, hunting of suction refrigerant gas flow tends to occur due to a mutual interference between the control of the variable displacement mechanism and the control of the expansion valve immediately after t.sub.1 shown in FIG. 9.